Over the years, various arrangements have been developed to effect vertical movement of a patient support portion of a hospital bed with respect to a base. For example, upward and downward movement of a patient support litter relative to a base has been effected with a pair of spaced hydraulic cylinders which have housings fixedly mounted on the base and which have vertically extending piston rods with their upper ends fixedly secured to the patient support portion. However, hydraulic arrangements tend to drip oil, which creates a mess and in some cases presents a safety problem when the oil ends up on a floor surface where someone may slip on it. Further, in order to be competitive in today's marketplace, a hydraulic arrangement must usually include both electrically and manually actuated pumps, because the convenience of electrical pumps is normally desired but manual pumps are necessary for emergency circumstances during an electrical power outage. However, the provision of both manual and electrical systems can render the resulting bed relatively complex and expensive.
One alternative approach is to use a lift mechanism which includes a vertically extending threaded member operatively engaging a nut, and includes a drive arrangement to effect relative rotation of the threaded member and nut in order to provide vertical movement of the threaded member relative to the nut. An example of such a mechanism is disclosed in copending U.S. Ser. No. 07/816,826, filed Jan. 3, 1992 now U.S. Pat. No. 5,172,442 . While this known mechanism has been a distinct improvement over prior approaches and has been entirely adequate for its intended purposes, it has not been satisfactory in all respects.
It is the object of the present invention to provide, for a hospital bed, an improved lift mechanism of the type which includes a threaded member engaging and rotatable relative to a threaded part such as a nut.
It is a further object to provide such a lift mechanism in which the noise generated by the lift mechanism during operation is minimal, and in which certain friction-producing forces are compensated at least in part by opposite forces in order to reduce friction and thus achieve increased efficiency.
It is a further object to provide such a mechanism in which the drive mechanism is operated by an electric motor and the electric motor is electrically isolated from all of the components of the lift mechanism in a simple and inexpensive manner.
It is a further object to provide such a mechanism in which an electrical cord signals for the lift mechanism is kept spaced from moving components of the lift mechanism in a tangle-free manner as the lift mechanism moves the patient support part vertically with respect to the base.
It is a further object to provide such a mechanism in which a simple and inexpensive arrangement is provided to generate an electrical signal indicative of the vertical position of the patient support part relative to the base.
A further object is to provide such a mechanism in which a reliable but simple manual drive arrangement is provided to permit manual operation of the lift mechanism.